SAN FRANCISCO — The value of Yahoo shares still depends more on moves made by founder Jerry Yang years ago than anything current CEO Marissa Mayer has done during her 30-month tenure.

That’s only going to change if Mayer and the Yahoo board do something drastic based on their discussions last week, like reversing course and scrubbing the company’s planned spin-off of its Alibaba stake. (That appeared to be the case late Tuesday, after CNBC reported that Yahoo is reconsidering the spin-off. A decision could come as soon as Wednesday.)

And Yang was the “Yahoo” who – along with Softbank chairman and early Yahoo investor Masayoshi Son — pulled the trigger on the company’s equity investment in Alibaba.

That $1 billion investment in 2005 is today worth $31.8 billion, based on a 15.3% stake indicated in Alibaba’s June proxy statement.

Given that all of Yahoo (YHOO) was worth $32.6 billion on Monday, Yang’s decision now accounts for essentially all of Yahoo’s value to shareholders.

Ironically, it was also Yang who, during a brief stint as CEO of the company from 2007 to January, 2009, led the opposition to Microsoft’s aggressive takeover attempt.

Microsoft offered $44.5 billion in February 2008, or 37% more than the company’s current valuation, even including the Alibaba stake.

Even though Microsoft sweetened its offer during the months-long battle, Yang and his fellow board members held fast on selling a Silicon Valley Internet icon to a company many in Northern California viewed then as Tech Industry Enemy No. 1.

From what on paper looked like a weak negotiating position, Yang led the charge to keep independent the company he and Jerry Filo co-founded in 1995 to categorize Web sites.

Yet despite all that corporate diligence – or rather, because of it -- Yahoo shareholders screamed a collective 'ouch'!

Yang stepped down less than a year after Microsoft was rebuffed.

Shareholders then suffered a long and painful drop in value from a stock that kept going down — until Mayer arrived in mid-2013.

While Yang cost Yahoo shareholders dearly in early 2008 by rebuffing Microsoft, he and Softbank’s Son had already offset some of the damage via the Alibaba investment.

During Mayer's tenure, Yahoo’s stakes in Alibaba (BABA) and Yahoo Japan (again with Softbank) have produced gains on equity investment that have kept Yahoo’s net income in the black.

That’s allowed Mayer to invest in her mobile-first plans, but that strategy has yet to matter much in relation to the Alibaba stake.

Longer term for Yahoo shareholders, Yahoo’s core business of selling digital ads is getting pushed toward irrelevancy by Google and Facebook.

What value there is from today forward will remain based on the decision of Yang and Softbank’s Son who saw the potential of what Alibaba co-founder Jack Ma was building in China.

What Alibaba built was one of the 10 most valuable tech companies in the world, now worth $208 billion.

John Shinal has covered tech and financial markets for more than 15 years at Bloomberg, BusinessWeek,The San Francisco Chronicle, Dow Jones MarketWatch, Wall Street Journal Digital Network and others. Follow him on Twitter: @johnshinal.